1. Field of the Invention
The present invention relates to an improvement in light emitting diodes and a method of making the same.
More particularly, the present invention concerns an improvement in light emitting diodes especially suitable for optical communications and a method of making the same.
2. Description of the Prior Art
Light emitting diodes have, in addition to their reliability, a superior characteristic in good linearity in input current vs. output light suitable for analogue transmission in optical communications. Light emitting diodes utilizing GaAs-GaAlAs crystal for 0.8 .mu.m band with a considerable efficiency and reliability have been already developed and are now in practical use in short distance and intermediate distance optical communications.
However, the most important problem in the practical optical communication system is optical coupling between the light emitting diode and the optical fiber, and in the conventional technique, the coupling part produces the largest loss in the optical circuitry.
FIG. 1 is a sectional side view showing one example of the conventional light emitting diode for optical communication, which comprises:
______________________________________ a substrate 1 of n-GaAs a first epitaxial layer 2 of 10.mu.m thick n-Ga.sub.1-x Al.sub.x As, (0 &lt; x .ltoreq. 1), a second epitaxial layer 3 of 1.5.mu.m thick p-GaAs (active layer), a third epitaxial layer 4 of 1.0.mu. m thick p-Ga.sub.1-x' Al.sub.x' As, (0 &lt; x' .ltoreq. 1), a fourth epitaxial layer 5 of 1.0.mu. m thick p-GaAs, an insulating film 6 of SiO.sub.2, a p-side electrode 7 of vacuum-deposited Au and an n-side electrode 8 of Au-Ge-Ni alloy. ______________________________________
The substrate has an opening 9 to expose a surface 21 of the first epitaxial layer 2 and an input end of an optical fiber 17 is disposed therein facing the surface 21.
Though having very high internal quantum efficiency, due to a high refractive index of the crystal forming the first epitaxial layer 2, almost all parts of the emitted light is are reflected towards the inside by the surface of the crystal, thereby lowering the external efficiency to only several %. Furthermore, since no structure is provided in the conventional light emitting diode to lead out the light like that in laser, the light of the conventional light emitting diode generally makes a wide distribution called "Lambertian". On the other hand, in an optical fiber the incident angle of light to be effectively transmitted is limited by the numerical aperture (NA) thereof, and therefore, due to the narrow incident angle the coupling efficiency from the light-emitting diode to the optical fiber is very small. Accordingly, in order to increase the overall input power to the optical fiber, in addition to the abovementioned increase of the external quantum efficiency, control of the distribution of the output light by a suitable measure is needed thereby to effectively lead the light into the optical fiber. Hitherto, for increasing coupling efficiency, such measures have been taken as to form the input ends of the optical fiber into a hemisphere or to insert a spherical lens between the light emitting diode and the input ends of the optical fiber. However, the former measure of spherical ended fiber coupling has a problem of difficulty in obtaining, in mass production, uniform hemisphere curvatures to which the coupling efficiency depends on, and the latter measure of inserting the minute spherical lens has a problem of difficulty in aligning its optical axis and fixing it in a right position.